text: Both the fuel and cladding can swell. Cladding covers the fuel to form a fuel pin and can be deformed. It is normal to fill the gap between the fuel and the cladding with helium gas to permit better thermal contact between the fuel and the cladding. During use the amount of gas inside the fuel pin can increase because of the formation of noble gases (krypton and xenon) by the fission process. If a Loss-of-coolant accident (LOCA) (e.g. Three Mile Island) or a Reactivity Initiated Accident (RIA) (e.g. Chernobyl or SL-1) occurs then the temperature of this gas can increase. As the fuel pin is sealed the pressure of the gas will increase (PV = nRT) and it is possible to deform and burst the cladding. It has been noticed that both corrosion and irradiation can alter the properties of the zirconium alloy commonly used as cladding, making it brittle. As a result, the experiments using unirradiated zirconium alloy tubes can be misleading.
  According to one paper the following difference between the cladding failure mode of unused and used fuel was seen.
  Unirradiated fuel rods were pressurized before being placed in a special reactor at the Japanese Nuclear Safety Research Reactor (NSRR) where they were subjected to a simulated RIA transient. These rods failed after ballooning late in the transient when the cladding temperature was high. The failure of the cladding in these tests was ductile, and it was a burst opening.
  The used fuel (61 GW days/tonne of uranium) failed early in the transient with a brittle fracture which was a longitudinal crack.
  It was found that hydrided zirconium tube is weaker and the bursting pressure is lower.
  The common failure process of fuel in the water-cooled reactors is a transition to film boiling and subsequent ignition of zirconium cladding in the steam. The effects of the intense hot hydrogen reaction product flow on the fuel pellets and on the bundle's wall well represented on the sidebar picture.